1. Technical Field of the Invention
This invention relates to electromechanical reciprocating rotary motion devices; and in particular to reciprocating angular motion actuators for producing constant amplitude, variable frequency, substantially triangular waveform motion profiles suitable for optical scanning applications.
2. BackGround Art
The use of an oscillating mirror and associated motor assembly is well known in the art for effecting a beam sweeping action. A characteristic of such devices, whether the motor producing the oscillation is a stepper motor or a galvanometer type motor, as is commonly the case, force, generated by current flowing through the motor windings must be used to decelerate the scanning motion and then reverse it. This necessarily generates heat, which is a significant problem in a very small device such as a galvanometer scanner. This heating, for a sinusoidal scan waveform, is proportional to the fourth power of scan frequency, and the square of the scan angle.       position    :          θ      ⁢              xe2x80x83            ⁢      sin      ⁢              xe2x80x83            ⁢      ω      ⁢              xe2x80x83            ⁢      t            velocity    :          ω      ⁢              xe2x80x83            ⁢      θ      ⁢              xe2x80x83            ⁢      sin      ⁢              xe2x80x83            ⁢      ω      ⁢              xe2x80x83            ⁢      t                  accelleration      :                        -                      ω            2                          ⁢        θ        ⁢                  xe2x80x83                ⁢        sin        ⁢                  xe2x80x83                ⁢        ω        ⁢                  xe2x80x83                ⁢        t              =          τ      j            τ    =                            iK          2                ⁢                  
                ∴        i            =                                    ω            2                    ⁢          θ          ⁢                      xe2x80x83                    ⁢          j                          K          2                          P    =                  i        2            ⁢      R            T    =                  T        case            +                                                  R              th                        ⁡                          (                                                                    ω                    2                                    ⁢                  θ                  ⁢                                      xe2x80x83                                    ⁢                  j                                                  K                  t                                            )                                2                ⁢                  R          coil                    
Many mechanical schemes have been employed to reduce the motor current and associated heat problem.
In Khowles U.S. Pat. No. 4,958,894, the excitation of an electromagnetic coil operating on a magnet at the end of a pivot arm extending off the mirror, is coordinated with the end-of-travel engagement of the magnet with one or the other of two resilient bumpers between which it travels, imparting a reversing bounce and resulting in the oscillation of the pivot arm and mirror. This bumper variation produces a faster reversal and lowers the required energy.
In Culp""s U.S. Pat. No. 5,066,084, there is disclosed a constant velocity scanning apparatus in which the mirror oscillations are maintained with end-of-travel piezo motion actuators in combination with end-of-travel, resilient xe2x80x9cenergy absorbing and releasing contactsxe2x80x9d analogous to the rubber bumpers of Khowles. Howe""s U.S. Pat. No. 3,678,308, illustrates another variation on an oscillating scanner that employs mechanical springs to provide an end-of-travel bounce in the oscillating motion of the mirror.
These all involve scanning systems with mechanical springs defining the end of travel, and demonstrate well the general idea that opposing springs can be employed to conserve energy within a mechanically oscillating device. They use varying geometries and may also use modified motor drive current schemes for a coordinated effect on reducing average motor current while maintaining a satisfactory output waveform of the device.
It is instructive to look at U.S. Pat. No. 5,424,632, as illustrative of a common moving magnet scanner. The ""632 FIG. 1 is described as a schematic view of a galvanometer used in a laser scanning system, illustrating the mirror, motor, and a position transducer. In the ""632 FIG. 2, torque motor 17 includes a magnetically permeable outer housing 28 that holds the stator 51 consisting of windings 31 on bobbin 50. Permanent magnet rotor 100 is rotably mounted within the stator. Stator windings 31 in the ""632 FIGS. 3 and 8 is the coil where the heat of concern is generated. This heat is dissipated radially through the device.
The achievable flux density of the stator magnet 27 as well as the resistivity of winding 31 are subject to fundamental material constraints. The achievable acceleration of this system is a function of the aspect ratio of the magnet (length to diameter) and proportional to 1/(magnet radius), to first order. This means that larger structures allow lower RMS (root mean square) acceleration. RMS acceleration is defined over the relevant thermal time constants. In other words, it is the maximum acceleration at which the device can be run without heat-induced damage and eventual failure. In theory, one can put an arbitrarily large stator current, ignoring demagnetizing of the magnet, for an arbitrarily short time, but when attempting to execute a repetitive waveform, the device would simply reach a certain steady state. FIGS. 1 and 2 of the ""632 disclosure are included herein as prior art FIGS. 9 and 10 respectively.
Another area of art which readers may find instructive is that of resonant scanners. These scanners use a more or less linear spring, and constitute a mechanical oscillator in which energy is continually converted back and forth between kinetic energy (stored in the rotating mass) and potential energy (stored in a torsional spring). These can achieve very high efficiencies, as the motor only has to supply system losses, but they have two fundamental constraints. First, the frequency is constrained to the resonant frequency of the system. The frequency can be tuned, to some extent, such as by changing the temperature of the spring or making other mechanical adjustments to the design. There are patents to this effect. Second, the mechanical output motion must be sinusoidal, or very nearly so.
Dostal""s U.S. Pat. No. 3,609,485, is a resonant torsional oscillator for optical scanning or other vibratory action at a high amplitude and constant rate. This patent is cited in many torsional resonant scanners, an example of which is Corker""s U.S. Pat. No. 3,642,344, Optical Scanner Having High Frequency Torsional Oscillator. The problem with all of the resonant torsional oscillators is that they give sinusoidal motion, and are essentially constant frequency devices, being tunable over a narrow range by varying temperature or otherwise varying the spring rate of the spring.
In summary, there remains room for improvement in the design and operation of bi-directional reciprocating galvanometer scanners and similar reciprocating motion devices to reduce power requirements and minimize heat generation through the use of design features that provide for passive energy conservation in the change of direction phase of motion.
The invention may be most simply described as a reciprocating rotary action actuator consisting of a motor coupled to a rotor and stator where the stator has a ring magnet and a pair of soft iron pole pieces that concentrate the flux of the ring magnet into a concentric set of narrow, uniformly spaced, axially oriented, magnetic flux fields intersecting the rotor""s field of travel. The rotor has small permanent magnets embedded in the periphery of a nonconductive, nonmagnetic rotor core, where the magnets are of the same number and spacing as the stator""s magnetic flux fields, there being at least one and preferably two or more with equal spacing. The magnets are pole oriented axially opposite the flux fields of the stator pole pieces, so that upon rotation, the rotor magnets encounter the stator flux fields at each end of rotor travel, creating an opposing force that reverses the angular direction of the rotor with minimal requirement for motor current. The device can be incorporated into a galvanometer scanner or other devices with similar reciprocating rotary action requirements.
More particularly, the invention encompasses a high speed reciprocating angular motion device, adaptable to electrically powered optical scanning and other applications where frequency, amplitude, load moment of inertia, and scan efficiency are generally limited by thermal considerations of the actuator. The limitations of the prior art are overcome by combining a bi-directional, electrical drive actuator for driving a reciprocating scanner rotor with high efficiency, while a preferably passive, energy transformation mechanism, the equivalent of a set of hi-K (spring constant) bumpers or springs, provides for decelerating, reversing and re-accelerating the rotor motion at each end of its arc of rotation. The passive reversing function is enabled magnetically, and may be modified in some embodiments to provide limited adjustment for tuning and matching of spring set characteristics.
The design is a radical departure from the prior art of scanner actuators. The design calculations for a device of the invention having the required passive or near passive ability for repeatedly reversing the rotor, motor and design load direction, and executing these acceleration changes within a specified small portion of the rotor arc of travel, with substantially little contribution from motor current or impact on thermal budget, are not trivial. However, it should pose no special problem for those skilled in the art, upon a full and careful reading and understanding of this disclosure and its priority document, which is hereby incorporated by reference.
The motion waveform of the device may be constrained to be triangular rather than sinusoidal. A triangular scan waveform is more useful in many cases, as the largely constant velocity aspect of the rotor motion is often easier to incorporate into component and system designs, and offers efficiencies over a sinusoidal scan. The frequency of the reciprocating motion is not constrained by the invention. What is constrained by the particular design of any embodiment is amplitude, the useful arc distance of rotor motionxe2x80x94this is a substantially constant amplitude system whose frequency can be varied at will.
Other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein we have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by us on carrying out our invention.